It is well known that the stomach plays a critical role in the digestion of foodstuffs taken in by the body by generating a solution that contains 0.16N hydrochloric acid. This role is essential for the normal survival of the organism. The conductive movement of H+ ions across the apical membrane plays a key role in acid secretion. The coupling of the net efflux of H+ ions with the recycling of K+ ions allows us to maintain the continued acid flux that occurs during stimulated acid secretion. In addition to this movement one expects to observe a parallel efflux of chloride to aid in the generation of acid from the gland lumen. The stimulated loss of acid occurs when the H-K-ATPase is actively inserted into the apical membrane causing a proton efflux from cell to lumen of the gland. What happens to the cells of the gland during the non-stimulated state is completely unknown due to the previous inaccessibility to the apical surface of the intact gland. The key focus of this grant will be to use the various techniques that have been developed in our laboratory to access the apical surface of the intact gland and to examine the biophysics and regulation of acid secretion transcellularly, focusing on the changes that develop from the non-stimulated gland to the stimulated gland. We will use our findings to explain what changes are developing in the stomach during both hyper- and hypoacidic states and to examine which of the major ions (Na+, Ca2+, K+, Cl-, HCO3) regulate, and/or stimulate the various phases of acid secretion. The results of this study will allow us to develop a new model of acid secretion including newly defined transport proteins that were inaccessible before the development of our various techniques. These data will help us to design strategies of control for both hyper- and hypoacidity.